The invention relates to filters for trapping solid particulates present in fluids and, in particular, to filter bodies formed from thin porous walled, honeycomb structures.
The removal of solid particulates from fluids, gases or liquids, in which the particulates are mixed or suspended is typically accomplished by means of filters made from solid materials which are formed into articles or masses having a plurality of pores of small cross-sectional size extending therethrough, which may be interconnected, such that the solid materials are both permeable to the fluids which flow through the article or mass and capable of restraining some or all of the particulates mixed in the fluid from passing through the article or mass, as desired. Such pores constitute what is termed "open porosity" or "accessible porosity". The restrained particulates are collected on the inlet surface(s) and/or within the pores of the material. The minimum cross-sectional size of some or all of the pores can be larger than the size of some or all of the particulates to be removed from the fluid, but only to the extent that significant or desired amounts of sufficiently large particulates become trapped on or within the filters during the transit of contaminated fluid. As the mass of collected particulates increases, the flow rate of the fluid through the filter generally decreases to an undesirable level. The filter is then either discarded as a disposal, replaceable element or regenerated by suitably removing the collected particulates so that it may be reused.
Certain considerations are helpful in evaluating and comparing filters. One is filter efficiency, that is, the amount of the suspended particulates of concern that are removed from the fluid as it passes through the filter (hereinafter expressed as a percentage of the weight of all particulates in the fluid prior to its passing through the filter). Required or desired efficiencies will vary for different filtering applications and typically will range from a substantial portion to all or substantially all of the particulates passed through the filter. Another consideration is flow rate, that is the volume of fluid per unit of time that passes through the filter and collected particulates. In a closed, continuous feed system, a consideration related to flow rate is the pressure drop across the filter, i.e. the difference between the fluid pressure upstream and downstream from the filter caused by the presence of the filter and particulates thereon which is dependent upon and increases with flow rate. Yet another consideration is operating time, the cumulative time of service of a filter before its flow rate or pressure drop becomes unacceptable so as to necessitate replacement and/or regeneration of the filter. Yet another consideration is particulate capacity, the amount of particulates material that the filter can hold while still providing a minimum acceptable flow rate or maximum acceptable pressure drop. Other desirable features of the filter include compact structure, mechanical integrity, thermal shock resistance durability, inertness or non-deleterious reaction with the fluid and/or particulate material.
It has been known for some time that honeycomb structures having a plurality of hollow passages or cells extending through them, which are formed in whole or in part by thin porous interconnected walls, may be successfully employed in filtering applications. U.S. Pat. Nos. 4,060,488, and 4,069,157 describe fluid filtering apparatus constructed by applying to a porous support body such as a thin porous walled honeycomb structure, a suitable filtering membrane. The membrane allows a separable component of a fluid solution or mixture transversing the honeycomb structure passages supporting the membrane to pass into the porous support structure. The filtrate fluid migrates through the open porosity of the support structure to an internal reservoir or to an outer surface of the structure where it is removed.
U.S. Pat. Nos. 4,041,591 and 4,041,592, assigned to the assignee hereof, describe multiple flow path bodies and methods for fabricating the same from honeycomb structures having columns or layers of hollow cells which extend in a substantially mutually parallel fashion through the structure and which are formed by a plurality of thin intersecting walls. The structure is modified so that two separate fluids may be transported through alternate columns or layers of cells. It is suggested that the described multiflow path body might optionally be used in filtration and osmotic separation applications by using porous materials to produce the original honeycomb structure. As described, the device would function like those of the aforesaid U.S. Pat. Nos. 4,060,488, and 4,069,157 which allow only a fraction of a fluid which is passed through the alternate columns or layers of cells to migrate across the thin porous walls into the adjoining interleaved columns or layers of cells while the remainder of the fluid, having a higher concentration of contaminant or separable constituent, continues to pass through and exits the structure.
Porous walled honeycomb structures may also be used directly (i.e. without a filter membrane or other covering) to filter all fluid passed through the structure. Application Ser. No. 165,646, filed July 3, 1980 and assigned to the assignee hereof, and U.S. Pat. No. 4,276,071 both describe filters formed from honeycomb structures, themselves formed by matrices of thin, porous, intersecting walls which define a pair of open, opposing end faces and a multiplicity of hollow, substantially mutually parallel passages or cells extending longitudinally through the structure between the end faces. At each end face where the open transverse areas of the cells would normally be exposed, the ends of alternate cells are closed in a checkered or checkerboard pattern such as is depicted in an exemplary fashion in FIG. 2. The pattern is reversed at either end face so that each cell of the structure is closed at only one end face and shares thin walls in common with adjoining cells which are blocked only at the opposing end face of the structure. A contaminated fluid is introduced under pressure to a "inlet" end face of the filter body and passes into those cells open at the inlet end face ("inlet" cells). Because the inlet cells are closed at the opposing, "outlet" end face of the structure, the contaminated fluid is forced to pass across the narrow dimension of the thin, porous walls into the adjoining "outlet" cells (which are all closed at the inlet end face and open at the outlet end face of the structure) and through them from the outlet end face of the filter. All or substantially all of the solid particulate matter is deposited on the thin wall surfaces defining the interior of the inlet cells or is trapped in the pores forming the open porosity of the thin walls. All of the contaminated fluid passing through the structure is filtered and all of the interconnected, thin walls of the structure, each of which is shared in common between an adjoining inlet and outlet cell, are utilized in filtering.
U.S. Pat. No. 4,276,071 also describes a second filter embodiment formed from a crossflow honeycomb structure in which layers of longitudinally extending and laterally extending passages are alternately stacked. Fluid containing solid particulate contaminant is passed into either end of one of the two commonly extending sets of passages and is recovered after filtering from the remaining set of interleaved, commonly extending passages. As only the thin walls separating the laterally and longitudinally extending layers of cells are porous, filtration occurs only through those thin walls which are shared in common between adjoining inlet and outlet passages. Accordingly, this device is only about one-half as efficient as a comparably sized and dimensioned honeycomb structure incorporating the previously described checkered pattern of alternate cells.
As is described and claimed in a copending application Ser. No. 350,998 entitled FILTER APPARATUS, filed on the same date as this application and assigned to the assignee hereof, by providing a filter of honeycomb structure formed by a matrix of interconnected thin walls having at least a minimum requisite amount of interconnected open porosity, filtration can be achieved through all thin walls defining each inlet cell irrespective of the type of adjoining cell (i.e. inlet or outlet) with which the inlet cell shares its defining thin walls.
According to yet another copending application Ser. No. 350,995 also filed on the same date of this application and assigned to the assignee hereof, improved particulate capacity and operating time, albeit at the expense of initially increased pressure drop, typically can be achieved in a honeycomb filter of given total thin wall surface area by providing significantly more collective inlet cell than collective outlet cell thin wall surface area. Again, interconnected thin walls forming the cells of the filter are provided with sufficient interconnected open porosity to allow the fluid to flow completely through and across any of their dimensions in any direction and to prevent at least a significant portion of the solid particulate contaminant from flowing either completely through or completely across the walls in any direction.